DOCSLIB.ORG

The Subdwarf B + White Dwarf Binary KPD 1930 + 2752, a Supernova Type Ia Progenitor Candidate

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
The Subdwarf B + White Dwarf Binary KPD 1930 + 2752, a Supernova Type Ia Progenitor Candidate 15th European Workshop on White Dwarfs ASP Conference Series, Vol. 372, 2007 R. Napiwotzki and M. R. Burleigh The Subdwarf B + White Dwarf Binary KPD 1930 + 2752, a Supernova Type Ia Progenitor Candidate S. Geier,1 S. Nesslinger,1 U. Heber,1 N. Przybilla,1 R. Napiwotzki,2 and R.-P. Kudritzki3 1Dr.–Remeis–Sternwarte, Institute for Astronomy, University Erlangen-Nuremberg, Sternwartstr. 7, 96049 Bamberg, Germany 2Centre of Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK 3Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA Abstract. The nature of the progenitors of type Ia supernovae is still under controversial debate. KPD 1930+2752 is one of the best SN Ia progenitor can- didates known today. The object is a double degenerate system consisting of a subluminous B (sdB) star and a massive white dwarf. Maxted, Marsh, & North (2000) conclude that the system mass exceeds the Chandrasekhar mass. This conclusion rests on the assumption that the sdB mass is 0.5 M⊙. However, recent binary population synthesis calculations suggest that the mass of an sdB star may range from 0.3 M⊙ to more than 0.7 M⊙. It is therefore important to mea- sure the mass of the sdB star simultaneously with that of the white dwarf. Since the rotation of the sdB star is tidally locked to the orbit the inclination of the system can be constrained. An analysis of the ellipsoidal variations in the light curve allows to tighten the constraints derived from spectroscopy. We derive the mass–radius relation for the sdB star from a quantitative spectral analysis. The projected rotational velocity is determined for the first time from high-resolution spectra. In addition a reanalysis of the published light curve is performed. The atmospheric and orbital parameters are measured with unprecedented accuracy. −1 In particular the projected rotational velocity vrot sin i = 92.3 ± 1.5 kms is determined. The mass of the sdB is limited between 0.45 M⊙ and 0.52 M⊙. The total mass of the system ranges from 1.36 M⊙ to 1.48 M⊙ and hence is likely to exceed the Chandrasekhar mass. So KPD 1930 + 2752 qualifies as an excellent double degenerate supernova Ia progenitor candidate. 1. Introduction Supernovae of type Ia (SN Ia) play an important role in the study of cosmic evolution. They are regarded as the best standard candles for the determina- tion of the cosmological parameters. The nature of their progenitors is still under debate. There is general consensus that only the thermonuclear explosion of a white dwarf (WD) is compatible with the observed features of SN Ia. A white dwarf has to accrete mass from a close companion to reach the Chan- drasekhar limit of 1.4 M⊙. Two main scenarios of mass transfer are currently under discussion. In the so-called single degenerate (SD) scenario (Whelan & Iben 1973), the mass donating component is a red giant/subgiant, which fills 393 394 Geier et al. its Roche lobe and is continually transferring mass to the white dwarf. Accord- ing to the so-called double degenerate (DD) scenario (Iben & Tutukov 1984) the mass donating companion is a white dwarf, which eventually merges with the primary due to orbital shrinkage caused by gravitational wave radiation. KPD 1930 + 2752 was identified as a pulsating subdwarf B star (sdBV). Multi- periodic variations with short periods and low amplitudes were detected. In addition to these pulsations a strong variation at a much longer period of about 4100 s was found. This variation could be identified as an ellipsoidal deformation of the sdB star most likely caused by a massive companion. Bill`eres et al. (2000) predicted the period of the binary to be two times the period of this brightness variation. This was proven by Maxted et al. (2000), who measured a radial ve- locity curve of KPD 1930 + 2752 which matched the proper period. The radial velocity amplitude combined with an assumption of the canonical mass for sdB stars MsdB = 0.5 M⊙ led to a lower limit for the mass of the system derived from the mass function. This lower limit M ≥ 1.47 M⊙ exceeded the Chandrasekhar mass of 1.4 M⊙. The system is considered to become double degenerate when the subdwarf eventually evolves to a white dwarf. Orbital shrinkage caused by gravitational wave radiation will lead to a merger of the binary. Maxted et al. concluded that KPD 1930 + 2752 could be a good candidate for the progenitor of a type Ia supernova. This conclusion rests on the assumption that the mass of the primary is 0.5 M⊙. SdB stars have been identified as core helium burning stars at the blue end of the horizontal branch. According to canonical stellar evolution the mass is fixed by the onset of the core helium flash to about half a solar mass (Heber 1986). However, binary population synthesis calculations (Han et al. 2003) suggest, that the mass range for sdB stars is much wider, ranging from 0.3 M⊙ to more than 0.7 M⊙. Therefore we aim at measuring the mass of the sdB star simultaneously with that of the white dwarf. Since the rotation of the sdB star is tidally locked to the orbit the inclination of the system can be constrained, if the sdB radius and the projected rotational velocity can be measured with high precision. An analysis of the ellipsoidal variations in the light curve allows to tighten the constraints derived from spectroscopy. We derive the mass–radius relation for the sdB star from a quantitative spectral analysis. The projected rotational velocity is determined for the first time from high-resolution spectra. In addition a reanalysis of the published light curve is performed. 2. Observations and Data Analysis With the 10 m Keck I Telescope at the Mauna Kea Observatory two hundred high-resolution spectra were obtained using the High Resolution Echelle Spec- trometer (HIRES). Two spectra were taken with the ESO Very Large Telescope UT2 (Kueyen) equipped with the UV-Visual Echelle Spectrograph (UVES). Ad- ditional 150 low-resolution spectra were obtained with the 2.2 m Telescope at the Calar Alto Observatory using the Calar Alto Faint Object Spectrograph (CAFOS). The data was reduced using the ESO–MIDAS package. The radial velocities were obtained by cross correlation with a model spectrum at rest wave- length. Combining with data from literature we derive: K = 341 ± 1kms−1, P = 0.0950933 ± 0.0000015 d. The sdB+WD Binary KPD 1930+2752 395 For the analysis of the spectra of KPD 1930+2752 we used LTE model grids for 10 times solar metallicity. As the surface gravity is of utmost importance for our analysis we also calculated new grids of models and synthetic spectra to account for NLTE effects and metal line blanketing simultaneously. The mean parameter values from the model atmosphere fits are: Teff = 35 200 ± 500 K and log g = 5.61 ± 0.06 dex. The primary aim of the high-resolution time-series spectroscopy was to measure the projected rotational velocity of KPD 1930 + 2752 as accurately as possible. For this purpose the spectra obtained with HIRES and UVES were used. The projected rotational velocity was measured by convolving a synthetic spectrum calculated from the best fit model atmosphere with a rotational broad- ening ellipse for appropriate vrot sin i (see Figure 1). The result is very accurate: −1 vrot sin i = 92.3 ± 1.5kms . 3. Mass and Inclination KPD 1930 + 2752 is obviously affected by the gravitational forces of the com- panion, demonstrated by its ellipsoidal deformation. Since the period of the photometric variations is exactly half the period of the radial velocity variations the rotation of the sdB star is tidally locked to the orbit. Having determined the gravity and projected rotational velocity, we have three equations at hand that constrain the system, with the sdB mass MsdB being the only free parameter. Besides the mass function 3 3 2 Mcomp(sin i) KsdBP f(MsdB,Mcomp)= 2 = (1) (MsdB + Mcomp) 2πG these are v P sin i = rotsini (2) 2πR M G R = sdB (3) s g With log g obtained from the model atmosphere analysis, the radius of the star R was calculated using the standard mass–radius relation (Eq. 3). Together with the orbital period of the system P and the projected rotational velocity vrotsini = vrot sin i the inclination of the system sin i was derived for different values of the sdB mass. Because the rotation is tidally locked to the orbit, the rotational period of the sdB equals the orbital period of the system. Therefore the absolute value of the rotational velocity could be calculated. From the measured projected rotational velocity the inclination of the system follows (Eq. 2). With the sdB mass as a free parameter, the measured radial velocity semi-amplitude KsdB and orbital period P the mass function was solved numerically (Eq. 1) to derive the mass of the companion Mcomp and calculate the total mass of the binary. The fact that sin i cannot exceed unity gave a lower limit for the mass of the sdB MsdB ≥ 0.45 M⊙. The total mass of the system exceeds the Chandrasekhar limit for almost all assumptions of MsdB. The inclination angle of the system is close to 90◦ implying that KPD 1930 + 2752 may be an eclipsing binary. 396 Geier et al. 1.00 1.80 113 113 112 1.70 112 0.98 eclipse limit 111 eclipse limit 111 110 sun 110 109 109 108 ) / M 1.60 108 0.96 107 comp 107 sin(i) 106 106 1.50 105 + M 105 χ2 χ2 sdB 0.94 (M 1.40 0.92 1.30 0.48 0.52 0.56 0.60 0.48 0.52 0.56 0.60 MsdB / Msun MsdB / Msun Figure 1.
Load more

Recommended publications
  • A Search for Radio Pulsations from Neutron Star Companions of Four
    Astronomy & Astrophysics manuscript no. 16098˙arxiv c ESO 2018 November 15, 2018 A search for radio pulsations from neutron star companions of four subdwarf B stars Thijs Coenen1, Joeri van Leeuwen2,1, and Ingrid H. Stairs3 1 Astronomical Institute ”Anton Pannekoek,” University of Amsterdam, P.O. Box 94249, 1090 GE, Amsterdam, The Netherlands 2 Stichting ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands 3 Dept. of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, B.C., V6T 1Z1 Canada ABSTRACT We searched for radio pulsations from the potential neutron star binary companions to subdwarf B stars HE 0532-4503, HE 0929- 0424, TON S 183 and PG 1232-136. Optical spectroscopy of these subdwarfs has indicated they orbit a companion in the neutron star mass range. These companions are thought to play an important role in the poorly understood formation of subdwarf B stars. Using the Green Bank Telescope we searched down to mean flux densities as low as 0.2 mJy, but no pulsed emission was found. We discuss the implications for each system. 1. Introduction Several such binary formation channels have been hypoth- esized. For an sdB to form, a light star must lose most of its The study of millisecond pulsars (MSPs) enables several types hydrogen envelope and ignite helium in its core. In these binary of research in astrophysics, ranging from binary evolution (e.g. systems, sdB stars can be formed through phases of Common Edwards & Bailes, 2001), to the potential detection of back- Envelope evolution where the envelope is ejected, or through ground gravitational radiation using a large set of pulsars with stable Roche Lobe overflow stripping the donor star of its hydro- stable timing properties (Jaffe & Backer, 2003).
    [Show full text]
  • White Dwarf and Hot Subdwarf Binaries As Possible Progenitors of Type I A
    White dwarf and hot sub dwarf binaries as p ossible progenitors of type I a Sup ernovae Christian Karl July White dwarf and hot sub dwarf binaries as p ossible progenitors of type I a Sup ernovae Den Naturwissenschaftlichen Fakultaten der FriedrichAlexanderUniversitatErlangenN urnberg zur Erlangung des Doktorgrades vorgelegt von Christian Karl aus Bamberg Als Dissertation genehmigt von den Naturwissenschaftlichen Fakultaten der UniversitatErlangenN urnberg Tag der m undlichen Pr ufung Aug Vositzender der Promotionskommission Prof Dr L Dahlenburg Erstb erichterstatter Prof Dr U Heb er Zweitberichterstatter Prof Dr K Werner Contents The SPY pro ject Selection of DB white dwarfs Color criteria Absorption line criteria Summary of the DB selection The UV Visual Echelle Sp ectrograph Instrumental setup UVES data reduction ESO pip elin e vs semiautomated pip eline Derivation of system parameters Denition of samples Radial velocity curves Followup observations Radial velocity measurements Power sp ectra and RV curves Gravitational redshift Quantitative sp ectroscopic analysis Stellar parameters of singleline d systems
    [Show full text]
  • EC 10246-2707: a New Eclipsing Sdb+ M Dwarf Binary
    Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 10 September 2018 (MN LATEX style file v2.2) EC 10246-2707: a new eclipsing sdB + M dwarf binary⋆ B.N. Barlow1,2†, D. Kilkenny3, H. Drechsel4, B.H. Dunlap2, D. O’Donoghue5,6, S. Geier4, R.G. O’Steen2, J.C. Clemens2, A.P. LaCluyze7, D.E. Reichart2,7, J.B. Haislip7, M.C. Nysewander7, K.M. Ivarsen7 1Department of Astronomy & Astrophysics, The Pennsylvania State University, University Park, PA 16801, USA 2Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255, USA 3Department of Physics, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa 4Dr. Karl Remeis-Observatory & ECAP, Astronomical Institute, Friedrich-Alexander University Erlangen-Nuremberg, Sternwartstr. 7, D 96049 Bamberg, Germany 5South African Astronomical Observatory, PO Box 9, 7935 Observatory, Cape Town, South Africa 6Southern African Large Telescope Foundation, PO Box 9, 7935 Observatory, Cape Town, South Africa 7Skynet Robotic Telescope Network, Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255, USA 10 September 2018 ABSTRACT We announce the discovery of a new eclipsing hot subdwarf B + M dwarf binary, EC 10246-2707, and present multi-colour photometric and spectroscopic observations of this system. Similar to other HW Vir-type binaries, the light curve shows both primary and secondary eclipses, along with a strong reflection effect from the M dwarf; no intrinsic light contribution is detected from the cool companion. The orbital period is 0.118 507 993 6 ± 0.0000000009 days, or about three hours.
    [Show full text]
  • A Search for Radio Pulsations from Neutron Star Companions of Four Subdwarf B Stars
    A&A 531, A125 (2011) Astronomy DOI: 10.1051/0004-6361/201016098 & c ESO 2011 Astrophysics A search for radio pulsations from neutron star companions of four subdwarf B stars T. Coenen1, J. van Leeuwen2,1, and I. H. Stairs3 1 Astronomical Institute “Anton Pannekoek”, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands e-mail: [email protected] 2 Stichting ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands 3 Dept. of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, B.C., V6T 1Z1, Canada Received 8 November 2010 / Accepted 29 May 2011 ABSTRACT We searched for radio pulsations from the potential neutron star binary companions to subdwarf B stars HE 0532-4503, HE 0929- 0424, TON S 183 and PG 1232-136. Optical spectroscopy of these subdwarfs has indicated they orbit a companion in the neutron star mass range. These companions are thought to play an important role in the poorly understood formation of subdwarf B stars. Using the Green Bank Telescope we searched down to mean flux densities as low as 0.2 mJy, but no pulsed emission was found. We discuss the implications for each system. Key words. stars: neutron – pulsars: general – subdwarfs 1. Introduction insensitivity of their survey to longer-period binaries, Maxted et al. (2001) conclude that binary star evolution is fundamental The study of millisecond pulsars (MSPs) enables several types to the formation of sdB stars. of research in astrophysics, ranging from binary evolution (e.g. Several such binary formation channels have been hypoth- Edwards & Bailes 2001), to the potential detection of back- esized.
    [Show full text]
  • Variable Star
    Variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) fluctuates. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either: Intrinsic variables, whose luminosity actually changes; for example, because the star periodically swells and shrinks. Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth; for example, because the star has an orbiting companion that sometimes Trifid Nebula contains Cepheid variable stars eclipses it. Many, possibly most, stars have at least some variation in luminosity: the energy output of our Sun, for example, varies by about 0.1% over an 11-year solar cycle.[1] Contents Discovery Detecting variability Variable star observations Interpretation of observations Nomenclature Classification Intrinsic variable stars Pulsating variable stars Eruptive variable stars Cataclysmic or explosive variable stars Extrinsic variable stars Rotating variable stars Eclipsing binaries Planetary transits See also References External links Discovery An ancient Egyptian calendar of lucky and unlucky days composed some 3,200 years ago may be the oldest preserved historical document of the discovery of a variable star, the eclipsing binary Algol.[2][3][4] Of the modern astronomers, the first variable star was identified in 1638 when Johannes Holwarda noticed that Omicron Ceti (later named Mira) pulsated in a cycle taking 11 months; the star had previously been described as a nova by David Fabricius in 1596. This discovery, combined with supernovae observed in 1572 and 1604, proved that the starry sky was not eternally invariable as Aristotle and other ancient philosophers had taught.
    [Show full text]
  • Variability of Hot Subdwarf Stars from the Palomar-Green Catalog of Ultraviolet Excess Stellar Objects
    ABSTRACT VARIABILITY OF HOT SUBDWARF STARS FROM THE PALOMAR-GREEN CATALOG OF ULTRAVIOLET EXCESS STELLAR OBJECTS This thesis presents a survey for variability in 985 objects identified as hot subdwarf stars by the Palomar-Green Catalog of Ultraviolet Excess Stellar Objects. Catalina Real-Time Transient Survey (CRTS) light curves are used to find the variability. Landolt standard stars that are also hot subdwarf stars in the Palomar-Green Catalog are used to calibrate CRTS photometry in the visual (V) band, and show it is accurate to within ΔV = 0.1 magnitudes between V = 12.0 and V = 16.0. Eleven objects are found to have variability that exceeds four standard deviations above the means of their CRTS light curves. Two are objects already known to be variable. They are PG 1101+385 (Mrk 421), a BL Lac object, and PG 1419+081, a short-period, pulsating sdB star. The other nine are new discoveries of variability: PG 0901+309, PG 0923+329, PG 0934+554, PG 1201+258, PG 1025+244, PG 1411+219, PG 1640+645, PG 1700+315, and PG 2217+059. PG 0934+554 is a spectrophotometric standard of Massey et al. (1988): this discovery of its variability shows that it should not be used as a standard. PG 0934+554 is identified as a BL Lac object. More tentative identifications include PG 1411+219 as an AM CVn star, PG 1640+645 and PG 2217+059 as dwarf novae, and PG 1700+315 as an eclipsing binary star system. Detailed follow-up observations are needed for the other variables, although they may be pulsating hot subdwarf stars.
    [Show full text]
  • Hot Subdwarf Binaries from the MUCHFUSS Project Analysis of 12 New Systems and a Study of the Short-Period Binary Population
    A&A 576, A44 (2015) Astronomy DOI: 10.1051/0004-6361/201425213 & c ESO 2015 Astrophysics Hot subdwarf binaries from the MUCHFUSS project Analysis of 12 new systems and a study of the short-period binary population T. Kupfer1,S.Geier2, U. Heber3, R. H. Østensen4,B.N.Barlow5,P.F.L.Maxted6, C. Heuser3, V. S chaffenroth3,7, and B. T. Gänsicke8 1 Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands e-mail: [email protected] 2 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany 3 Dr. Karl Remeis-Observatory & ECAP, Astronomical Institute, Friedrich-Alexander University Erlangen-Nuremberg, Sternwartstr. 7, 96049 Bamberg, Germany 4 Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Heverlee, Belgium 5 Department of Physics, High Point University, 833 Montlieu Avenue, High Point, NC 27262, USA 6 Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK 7 Institute for Astro- and Particle Physics, University of Innsbruck, Technikerstr. 25/8, 6020 Innsbruck, Austria 8 Department of Physics, University of Warwick, Coventry CV4 7AL, UK Received 24 October 2014 / Accepted 12 January 2015 ABSTRACT The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims at finding hot subdwarf stars with massive compact companions like massive white dwarfs (M > 1.0 M), neutron stars, or stellar-mass black holes. The existence of such systems is predicted by binary evolution theory, and recent discoveries indicate that they exist in our Galaxy. We present orbital and atmospheric parameters and put constraints on the nature of the companions of 12 close hot subdwarf B star (sdB) binaries found in the course of the MUCHFUSS project.
    [Show full text]
  • Mode Identification in the Pulsating Subdwarf B Star KIC 2697388 A.S
    ACTA ASTRONOMICA Vol. 62 (2012) pp. 179–200 Mode Identification in the Pulsating Subdwarf B Star KIC2697388 A.S. Baran Mt. Suhora Observatory of the Pedagogical University, ul. Podchor ˛azych˙ 2, 30-084 Cracow, Poland email: [email protected] Missouri State University, Department of Physics, Astronomy, and Materials Science, 901 S. National Av., Springfield, MO 65897, USA Received June 28, 2012 ABSTRACT We present our results on mode identification in the pulsating subdwarf B star KIC2697388 observed with the Kepler spacecraft. We detected 148 frequencies of which five were attributed to p-modes while 122, to g-modes. The remaining 21 frequencies are also likely g-modes. We used multiplets and asymptotic period spacing to constrain degrees of 89 peaks to either l = 1 or 2. Using splittings in multiplets we derived the rotation period of this star to be nearly 45days. The average period spacing between l = 1 and 2 overtones are nearly 240 s and 139 s, respectively. Our results show that combining tentative identification based on the presence of multiplets and asymptotic period spacing is indeed useful in mode identification. Mode degree consistently inferred from independent methods make the results reliable and will help to construct accurate models of subdwarf B stars. Key words: subdwarfs – Stars: oscillations – Asteroseismology 1. Introduction Subdwarf B stars (sdB) are located on the blue extension of the horizontal branch. They are compact objects of half a solar mass, with typical radii of 0.2 R and typical effective temperatures of 30000K. We infer that an sdB progenitor on⊙ the main sequence has a mass similar to our Sun.
    [Show full text]
  • The Nature of Subdwarf B Stars U
    were able to satisfactorily fit the CIV emission profile at all - the absorbing clouds cover the variable continuum source epochs by varying only two parameters, namely the intensity and the broad emission line region; of the two broad Gaussian components. We found it reason­ - taking into account the fact that the non-variable continuum able to try a similar model in the case of NGC 4151. In this source is not covered by the absorbing cloud, the equiva­ case, the widths of the two resolved components are a = 7 and lent width of all the absorption lines is constant. 15 Arespectively. With this model, we have a good fit for all epochs. Indeed the fit included an unresolved Gaussian Acknowledgements component in absorption for which we were able to get the depth at each epoch. We have plotted the depth of the It is a pleasure to thank W. Huchtmeier and O. Richter for absorption component versus the strength of the underlying obtaining the 21 cm spectrum in the direction of NGC 4151. component; these two quantities are more or less propor­ We are grateful to E. Zuiderwijk for putting at our disposal his tional, but with a rather large scatter; we obtained a much line-fitting computer programme. better correlation when plotting the depth of the absorption line versus the total intensity of the emission (continuum plus the two broad emission components) at the wavelength of the References absorption line (Fig. 6). The CIV equivalent width is found to be 1. Seyfert, C. K. 1943, Astrophysical Journal 97, 28.
    [Show full text]
  • Physical Parameters of Sdb Stars from Spectral Energy Distributions?
    A&A 368, 994–1005 (2001) Astronomy DOI: 10.1051/0004-6361:20010068 & c ESO 2001 Astrophysics Physical parameters of sdB stars from spectral energy distributions? R. Aznar Cuadrado and C. S. Jeffery Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland e-mail: [email protected] Received 21 July 2000 / Accepted 4 January 2001 Abstract. The atmospheric parameters of 34 hot subdwarf B stars have been obtained using a combination of 61 short and long-wave IUE spectra, together with new and existing optical and infrared photometric data. Using a grid of high-gravity helium-deficient model atmospheres and a χ2-minimization procedure, 15 single sdB stars and 19 composite systems, containing a hot subdwarf B star and a cool main-sequence companion, were analyzed. From the (log L–log Teff ) diagram of our results, we conclude that the majority of the cool companions to our sample of binary sdB stars are main sequence stars, in the range of 4000 < Teff / K < 6000 and mass 0.8 <M/M < 1.3. The lower limit on detectability of cool stars in composite sdB of our sample is ∼3600 K, corresponding to a spectral type of M1 or later (Lang 1992). Key words. stars: formation, early-type, subdwarfs, fundamental parameters (luminosities, temperatures) – binaries: spectroscopic 1. Introduction have been claimed (Bixler et al. 1991). The point in study- ing companions to hot subdwarfs is that we can learn Subdwarf B (sdB) stars are considered to be core helium things about the hot subdwarfs that are not easy to ob- ∼ burning stars of mass 0.5 M with a very thin hydrogen- serve directly due to the problems of modelling hot atmo- < rich envelope ( ∼0.01 M , Heber 1986).
    [Show full text]
  • EVR-CB-001: an Evolving, Progenitor, White Dwarf Compact Binary Discovered with the Evryscope
    The Astrophysical Journal, 883:51 (12pp), 2019 September 20 https://doi.org/10.3847/1538-4357/ab3727 © 2019. The American Astronomical Society. All rights reserved. EVR-CB-001: An Evolving, Progenitor, White Dwarf Compact Binary Discovered with the Evryscope Jeffrey K. Ratzloff1 , Brad N. Barlow2 , Thomas Kupfer3 , Kyle A. Corcoran2,4, Stephan Geier5, Evan Bauer6 , Henry T. Corbett1, Ward S. Howard1 , Amy Glazier1, and Nicholas M. Law1 1 Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255, USA; [email protected] 2 Department of Physics and Astronomy, High Point University, High Point, NC 27268, USA 3 Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA 93106, USA 4 Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA 5 Institut für Physik und Astronomie, Universität Potsdam, Haus 28, Karl-Liebknecht-Str. 24/25, D-14476, Potsdam-Golm, Germany 6 Department of Physics, University of California, Santa Barbara, CA 93106, USA Received 2019 June 18; revised 2019 July 23; accepted 2019 July 29; published 2019 September 19 Abstract We present EVR-CB-001, the discovery of a compact binary with an extremely low-mass (0.21 ± 0.05Me) helium core white dwarf progenitor (pre-He WD) and an unseen low-mass (0.32 ± 0.06Me) helium white dwarf (He WD) companion. He WDs are thought to evolve from the remnant helium-rich core of a main-sequence star stripped during the giant phase by a close companion. Low-mass He WDs are exotic objects (only about 0.2% of WDs are thought to be less than 0.3 Me), and are expected to be found in compact binaries.
    [Show full text]
  • An Analysis of Pulsating Subdwarf B Star EPIC 203948264 Observed During Campaign 2 of K2
    EPJ Web of Conferences 160, 04007 (2017) DOI: 10.1051/epjconf/201716004007 Seismology of the Sun and the Distant Stars II An Analysis of Pulsating Subdwarf B Star EPIC 203948264 Observed During Campaign 2 of K2 Laura Ketzer1, and Mike Reed1, 1Department of Physics, Astronomy and Materials Science, Missouri State University, 901 S. National, Springfield, MO 65897, USA Abstract. We present a preliminary analysis of the newly-discovered pulsating subdwarf B (sdB) star EPIC 203948264. The target was observed for 83 days in short cadence mode during Campaign 2 of K2, the two-gyro mission of the Kepler space telescope. A time-series analysis of the data revealed 22 independent pulsation frequencies in the g mode region ranging from 100 to 600 µHz (0.5 to 2.8 hours). The main method − we use to identify pulsation modes is asymptotic period spacing, and we were able to assign all but one of the pulsations to either = 1 or = 2. The average period spacings of both sequences are 261.34 0.78 s ± and 151.18 0.34 s, respectively. The pulsation amplitudes range from 0.77 ppt down to the detection limit at ± 0.212 ppt, and are not stable over the duration of the campaign. We detected one possible low-amplitude, = 2, rotationally split multiplet, which allowed us to constrain the rotation period to 46 days or longer. This makes EPIC 203948264 another slowly rotating sdB star. 1 Introduction and hybrid pulsators, with both types of pulsations have intermediate temperatures. For a pre-Kepler pulsation The K2 mission is the extension of the very successful review, see Østensen[4].
    [Show full text]